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Файл:The bases of special methods of biomass processing into prospective materials. Tutorial
.pdf
Basic requirements for the processes are:
– temperature and its exposure time, defining rate and degree of
completeness of chemical processes;
– pressure, providing sufficient molding degree for creating required
contact between separate wood particles;
– a certain amount of water that plays an active role in processes
occurring during plastic formation.
Absence of at least one of the above mentioned conditions makes it
impossible to form LCWP.
Technological lines that produce material called lignoplast (lignum
means wood) using thermodynamic method work in Germany. Lignoplast
made of beech sawdust possesses good characteristics.
A method that allows obtaining composite materials (wood-particle
boards’ analogues) from any raw material of plant origin with no binding
agents has been developed by the scientists from Altai. The basic material
for them can be any vegetation as forestry residues and agricultural wastes
like bark, wood particles, sunflower pellets, nutshell, straw, etc. Moreover
they can be used separately and in combinations in order to reach required
characteristics. In order to obtain panel materials without binding agents a
perspective and widely used technology of water-thermal treatment is
employed. This technology is based on the method of explosive auto
hydrolysis, “steam explosion”, that allows selective affecting
lignocarbohydrated complex of plant tissue. When working with saturated
steam wood physical-mechanical and chemical changes occur at high
temperature. These changes are followed by destruction of
major components constituting wood structure, destruction of
morphological structure, hydrolysis of lignin and hemicelluloses with
formation of low-molecular instant products like pentosans and hexosans
(reducing substance).
When heated reducing substances undergo a reaction of
polycondensation with low-molecular lignin particles and become binding
agents in obtained material, constituting natural thermoplastic resin.
«Masonite» used in North America is considered to be an analogue
to this kind of material. Masonite being a type of FB was invented by
William H. Mason. Technology of its production is based on using plant
tissue explosive auto hydrolysis. Masonite compressing can be “wet” or
“dry”. However, resin and chemical additions, enhancing durability, waterresistance and other mechanical characteristics are used in masonite
production. In Europe this product is also known as Isorel.
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Engineering implementation was given to bioplastics. Cultural
medium of fungus is applied for gluing them. The system of chip
processing has been developed based on the use of two belt conveyors. The
chips, moving along the first conveyor, are subjected to steam sterilization.
Moving along the second conveyer they are cooled down by purified air
and treated by a special suspension, containing fungus and food regulators.
After the following procedures chips are put into clamps where they
undergo the two-week fermentation. Lignin-destroying fungi form around
the developing mycelium xylostroma, which upon further pressing produces
enhanced mechanical properties.
2.3. Wood modification
There are physical and chemical ways of modifying wood. Physical
methods lead to the changes in anatomical structure of wood. The process
occurs due to reduction of relative volume content of a wood lumen by
using cross-grain press-molding or filling cavities with inert materials.
Physical methods of wood modifying can be relatively divided into
two groups. The 1st group includes ways to improve the physical and
mechanical parameters of wood due to its compaction. There is a direct
proportion between wood density and its strength. This proportion is the
base of physical modifying and assumes wood molding. The consequence
of outer pressure is wood molding that occurs because of the reduction of
space between cells, molding and bending of cell walls and reduction of
cell. It is vitally important that the following change occur due to
deformation of cell elements but without their destruction.
According to the second group of physical methods wood cavities are
filled with materials that do not react with wood. This kind of chemical
treatment is used in order to provide wood with special predetermined
qualities with regard to further exploitation of wooden items.
There are the methods of mineral oil preimpregnation. As a result
self-lubricated antifriction material is obtained. A great effect is obtained
when wood is impregnated in polyethylene- and fluoroplastic-modified oil.
Wood metallization also belongs to this group of wood modifying.
This method implies placing of metal into wood cavities. The material
obtained can compete with metal when used for producing details for
machine-building industry.
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Insertion of high-boiling compounds like petrolatum and
polyethylene glycol into wood gives an opportunity to reduce its
penetrability by filling its pores.
Chemical modifying of wood can be defined as wood modification
with chemical substances. The results of such modification are
physicochemical changes in cell walls’ material. Depending on the nature
of the processes chemical modifying can be divided into following groups:
Acetylation is incorporation of acetyl groups into chemical
composition of wood. Acetic anhydride, ketone and some other substances
are used as acetylating substances. Acetyl wood has better decorative
appearance than natural one. In case of normal operating conditions
strength properties of this type of wood change insignificantly. But in case
of high humidity its mechanical qualities can be lower than of natural one.
Polymer-modified wood. This material looks like natural wood. Any
wood can be used for technology of this type. But a special technological
regime is developed for each technology. Workpieces dried up in ordinary
way with residual moisture of 2–10 % are required. Before being polymermodified workpieces undergo long and mild hydrothermal treatment. Low
humidity is required for successful polymer-modification as moisture,
preserved in wood structure, prevents its complete impregnation. But аt
high temperature during polymerization moisture boils and turns into steam
that supplants polymer, reducing physical-mechanical properties of
polymer-modified wood (PMW). In this case there will be no complete
polymerization.
Bulk impregnation with a high penetration polymer having a density
less than water takes place after wood humidity reduction. This becomes
possible due to hygroscopic property and porosity of wood. This polymer
can be applied in organic glass, safety glass (triplex), etc. Penetrating into
wood as water gets inside a sponge polymer turns into a solid state
(polymerize) under the influence of outer energy source.
Wood plays the role of reinforcement and its pores are filled with
polymer, covering cellulose fiber. That leads to formation of a solid
structure. The result is polymer-modified wood (PMW) that looks like
natural wood. It does not rotten as it is water-resistant surpassing larch.
Being similar in appearance to natural wood PMW has advantages of
plastic. At the same time it remains an ecologically pure low-flammable
product that possesses high heat-insulating and dielectric qualities. It can be
machined.
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After polishing, PMW has an ideally smooth surface, where natural
beauty of wood is visible. No additional treatment like painting, surfacing
or polishing is required. Being capillary-porous material ordinary wood has
an ability to return moisture and absorb steam from air depending on
atmospheric humidity and temperature and wood humidity and temperature.
That is why wooden goods can deform, resize, decay and conk.
2.4. Technology for obtaining the wood-polymeric composites
Wood-polymeric composite is a substance containing polymer
(chemical or natural) and chemically modified wood filler. Woodpolymeric composites have other widely-used names like “liquid wood”,
wood-plastic composite, wood thermoplast, WPC, EWFP (ecological wood
filled plastics), WFP (wood filled polypropylene). The peculiarity of woodpolymeric composites is that finished products are made of production and
consumer waste such as sawdust, shavings, flour wood, agricultural and
other kinds of wastes. Technologies that give an opportunity to use
secondary polymers instead of primary in manufacturing are being
developed.
WPCM consist of wood flour (or wood chips) with fraction of 0.5–2
mm, plastic binder and additives. PVC (50/50), polyethylene (70/30) or
polypropylene (60/40) can be used as polymer. To control industrial process
and WPCM properties modifying additives like antioxidants, antimicrobial
agents, surface-active substances, binding agents, shock-resistant modifiers,
lubricants, temperature stabilizers, coloring agents, fire-proof agents, light
stabilizers and foaming agents are used.
2.4.1. WPC classification
Wood-polymeric composite materials are divided according to:
1. Type of processing technology. According to this type there are:
– casting;
– extrusion;
– pressing;
– rolling.
2. Origin of cellulose raw material. According to this type they can
be made of:
– wood and wood wastes;
– other plants and plant wastes.
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3. The aggregated state of wood component. There can be:
– wood flour and sawdust (of fine fraction up to 200 μm, medium
fraction – 400 μm, coarse fraction of more than 400 μm);
– fiber (pine, deciduous, plant fiber).
4. Type of binding agent (matrix):
4.1. According to chemical type of matrix material:
– material, based on natural binding agents;
– material, based on artificial agents (thermoreactive and
thermoplastic);
– material, based on combined binding agents.
4.3. According to the initial state of matrix material they can be:
– liquid (solution, emulsion, dispersion);
– solid fine-dispersed (powder);
– solid, shaped (e.g. film or sheet).
5. According to chemical nature of polymeric matrix they can be:
– composites based on thermoreactive binding agents (e.g. polyether,
epoxy, phenol-formaldehyde, etc.);
– composites based on thermoplastic binding agents (e.g. polyamide,
polyolefine, polyester, etc.).
While composite processing (hardening) thermo reactive binding
agents become insoluble that enables recycling process. After processing
(moldings items from composites) thermoplastic binding agents practically
do not change their chemical structure. That makes complete recycle of
PCM possible without any substantial decrease of product characteristics.
6. According to strength WCM can be:
– nonstructural (heat-insulating, sound-insulating);
– weak (composite resistance is lower than that of wood);
– strong (as wood);
– high strength (higher than that of wood);
– extra-strong (much higher than that of wood).
7. According to construction WCM can be:
– single-layer;
– two-layer;
– multilayer.
8. According to moisture- and light-resistance, biological and other
external actions WCM are divided into:
– nondurable, including ones with high biodegradability, e.g. for
packaging;
– interior including:
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– not waterproof;
– moisture-resistant;
– moisture bio-resistant;
– for outdoor usage:
– moisture-resistant,
– water-resistant,
– extra-strong (contact with ground, mineral solutions, etc.);
– with special properties, e.g. – electro-insulating, bactericidal, etc.
9. According to surface type:
– unfinished;
– finished (polishing, scratch-brushing, embossing, printing);
– with polymeric layer coated in the process of production, e.g. using
co-extrusion method;
– finished with coating compositions;
– faced with natural wood veneer, synthetic films or metallized.
10. According to combustibility:
– combustible;
11. According to density (kg/m3):
– light, up to 900;
– dense, more than 900 up to 1200;
– heavy-weight, up to 1200.
WCM based on thermoplastic polymers are classified:
1) According to the type of binding thermoplastic polymer WCM can
be:
– based on artificial resins;
– based on biopolymers;
– based on mixture of artificial resins and biopolymers.
WCM based on artificial resin can be based on:
– polypropylene;
– polyethylene;
– polyurethane;
– polyvinylchloride;
– polystyrene.
2) According to purpose:
– structural (where they are used);
– heat-insulating;
– sound-insulating;
– with special properties.
3) According to the type of filer:
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– shavings;
– sawdust;
– wood flour;
– cellulose fiber (pulp-and-paper waste);
– nanoparticles.
4) According to density (kg/m3):
– ultra light, up to 700;
– light, from 700 up to 900;
– dense, from 900 up to 1200;
– heavy-weight, from 1200 up to 1400.
2.4.2. WPC content on the basis of thermoplastics
Not only pine wood as it is required in WBPL production but any
type of wood and wood wastes (like lath, cuttings, veneer sheets, shavings
and sawdust) and unclaimed wood obtained during forest sanitation.
Moreover, cellulose-containing wastes of farming industry (lint boon, rape,
rice husk, etc and self-renewable plants like rush, cane, etc.) can become
additional source of raw material.
In spite of the fact that wood is not a technological material there are
few requirements concerning vegetable fillers. Continuous WPC production
process is possible when structural and composition homogeneity of
vegetable filler is obtained. To obtain the filler of the required quality
hammer and impeller breaker mills, atomizers, rollers, pin-disk mills, etc
are employed. Particle’s size can differ from 0, 001 to 1 mm. The bigger the
filler particles the lower is the cost of the finished product. Coarse fractions
have limited usage as the size of particles is tightly connected with strength
and surface quality of the product obtained. Composite made of coarse
particles will have a more grained surface like wood particle board. That
may require polishing, thick siding and (or) topping of a surface. Fine flour
particles (less than 50 μm) have bigger specific surface area and due to this
fact require more resin to form a full-fledged polymeric matrix.
Apart from polymers wood cannot be plasticized in extruder or
injection-molding machine. It starts to burn at high pressure and
temperature. In addition to all the mentioned factors it has abrasive
properties. Highly porous structure of sawdust causes a number of serious
problems:
– sawdust absorbs gases and then releases them in extruder;
– obtained gas mixture is flammable and highly explosive;
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– pine wood gum while sawdust are processed.
Different groups of additives, depending on the formula, are used to
solve these problems.
Binding agent is рolypropylene (PP) according to State standard
26996-86, Technical Standards 2311-051-05796653-99.
Additives are:
– lubricants (external and internal) help prevent gluing to extrusion
nozzle, increase flow characteristics and reduce viscosity;
– binding agents and dispersants are combined with wood to enhance
dispersion in polymeric matrix. They also improve such mechanical
properties as breaking strength and bending strength, as well as shock
strength. They intensify durability by reducing water absorption and
deformation;
– practical experience has proved that usage of different
compatibilizers (substances that improve adhesion of mixture components)
is rational in production of polyolefine based WPC;
– foaming agents help get appearance and machinability that are
close to characteristic of natural wood. This is obtained because of WPC
product weight reduction. The following additives make nailing, drilling,
twisting-in and cutting easier;
– processing additives are used to prevent gluing to extrusion nozzle
and to improve characteristics of extruded article processing. Particularly it
is done to improve strength, elasticity and appearance of composition. They
also control thickness of foamed PVC walls;
– antioxidants are traditionally employed in preventing material
quality loss while processing and its ageing while usage. Heat stabilizers
minimize PVC quality loss while processing and material ageing while
years of usage;
– impact modifiers stiffen WPC based on PVC, ABS, etc and give
them reliability. These alloys improve weather ability properties and
resistance to scratches and damage;
– light (UV) stabilizers and pigments protect WPC products from UV
rays influence and prevent bluing;
– fungicides protect WPC products from microorganisms and mold.
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2.4.3. Manufacturing techniques
Manufacturing process and operation modes are defined by polymer
type and kind of obtained product.
General scheme of plastics industry includes traditional processes:
component processing, dosage, mixing of polymeric composition, product
molding and preservation of shape and physical-mechanical properties.
Preparation of compositions occurs in mixer of different types.
Turbulent and screw mixers are used for making dry mix. Calandering is a
specific and widely used method of polymeric compositions preparation.
The choice of molding method depends on the type of obtained
output. Sheet materials are usually molded in calenders; tubes and trim
moldings are extruded; piece works are injected.
Injection molding. Granules of thermoplastic polymer are bunkered.
Through a bin hopper they are loaded into cylindrical cavity of injection
machine, where high predetermined temperature is maintained due to
electric heating. Periodically working plunger injects softened material into
demountable cooled press-molds.
Molded items are released from press-molds and taken to storage
areas. This method is employed in production of small size, e.g.
polystyrene facing tile.
While molding liquid composition or polymer melt is put in molds
and harden due to polymerization, polycondensation and cooling.
Made by injection molding wood-filled composites are more
expensive than extrusion composites as injection molding requires
compound premixing and material predrying.
The type is wood is of primary importance in injection molding. At
the same time extrusion is more flexible in that case. It necessary to know
exact wood location parameters. Oak is sheared and burned faster than pine.
There can be a successful molding result with 60 % of pine, and not with
60 % from oak.
Drying of wood-filled granules is of greater importance in injection
molding than in extrusion as ventilation is usually not used in injection
processes. Absence of ventilation causes corrosion. Oak flour rich in tannic
acid causes press-molds corrosion, at the same time pine flour causes no
corrosion. Maple is characterized by the lowest unwanted gas emission. But
terpenes in pine exude tar. Pine is also easily coated but its flexural
modulus is lower than that of oak.
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Calendering. The initial raw material mixture consisting of
thermoplastic resin, plasticizers and other components are plasticized in
preheated rolls after being carefully premixed in heated mechanical mixers.
After that they are formed into roll materials with the help of special
machines — calenders, a press consisting of shafts. One- and two-layered
materials are made with the help of this method.
Materials are hot before they are put on heating rollers. That
procedure allows avoiding such deformations as curling and bending. These
deformations occur when materials do not accumulate enough heat. As a
result belt edges are chilled and only its medium part is pushed as heavy
load to preheated rolls. Consequently there can be buckles. Moreover,
molded products’ composition can be heterogeneous that can cause curling
and bending.
Mechanical anisotropy that is called calender effect is a specific
feature of calender formed products. This effect happens because of
orientation of polymer particles in direction of calendaring and is estimated
by difference in durability along and across a sheet.
Pressing in a mold. Mold pressing has been used for wood composite
processing for some time. Although for the majority of molded components
a mixture of natural fiber and wood as one of the options is used. During
compression molding, a press powder consisting of powdered thermosetting
resin and crushed filler is fed into a heated mold. Molding powder softens
and fills under pressure press-mold’s cavity. Hardening process occurs here.
Sanitary fittings and electrical equipment, window and door furniture,
architectural hardware, parts for construction equipment and machinery are
made in press-molds.
For flat pressing of constructing plastic sheets panel products
hydraulic multiopening presses with capacity of 100–500 cN heated with
superheated water or steam are used. Work piece assemblies are divided by
metal plates and put into a press. Glueing of particles into a product occurs
at 140–160 0С in a compressed state. Wood-particle boards, paper-based
laminates and plywood are obtained with the help of flat pressing.
Mold pressing has lower discharge capacity comparing to other
techniques. But the advantage is that fibers do not brake when being
pressed in a mold. When mold pressing higher concentrations than in
injection molding can be used. One of the companies in Arkansas that
implements mold pressing produces handles and other small parts using up
to 80 % of HDPE wood. Mold pressing can be easily adapted for different
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